purple beans for dinner; or, anthocyanin pigmentation in Phaseolus vulgaris

We had purple beans for dinner, from farmer's market.  I heated some olive oil in a pan and cooked them through until they had turned all green, and then salted them a little at the end.

The recipe called for adding chopped almonds but it didn't happen; instead I served the boys tofu that I had cut into cubes and boiled (boiling it improves the texture quite a bit, firming it up more to my liking).

I don't know which variety of purple beans we had.  A search of the net unearthed the "romano purpiat bean" (from Territorial Seed)

60 days. Everything about this purple Romano bean is noteworthy: healthy robust plants, deep violet stems, lilac blossoms, and lustrous purple pods. The 5 inch, flat pods are so tender, crisp, and delicious that we found them irresistible picked fresh from the plant. Cooking the pods turns them a brilliant jade-green with a nearly stringless texture. Remarkably productive plants reach 24 inches tall and tolerate cool, early plantings. Brown seeds.

and Peaceful Valley has the Royalty Purple variety

Phaseolus vulgaris Tender Annual Bush. Purple bushes with short runners and purple flowers. Bright-purple stringless 5"-6" pods cook to dark green. Buff colored seeds germinate in cold, wet soil. Bred by E.M. Meader at the University of New Hampshire and introduced in 1957.

Why are beans purple?  Good question.  I got as far as finding this:

Genetics of flower and pod color in Phaseolus vulgaris

Christian A. Okonkwo, and Carl D. Clayberg
The Journal of Heredity 1984:75(6):440-444
© 1984 The American Genetic Association 75:440-444

A new locus, Prp (purple pod), having five alleles affecting anthocyanin pigmentation of corolla and pod, is described in Phaseolus vulgaris L. The allele Prp produces dark-purple corolla and is fully dominant in this respect over the other four alleles that determine light-purple corolla. In the absence of Ro, Prp is responsible for medium-purple pod when homozygous and light-purple pods in combination with the other alleles. The alleles prp^sh2 and prp^sh give green pods shaded with purple and are codominant with the prp^st allele, which causes green pods to be striped with purple. These four alleles are dominant for pod color over prp, an allele causing green pods when homozygous. The interaction of these alleles with the genes Gri, V, and Ro, which also affect anthocyanin pigmentation of corolla and pod, is described. No linkages among these four loci were observed.

That's from 1984; I don't know enough about bean genetics to go any further.

(Oh, and they are delicious.)